The influence of cell growth and enzyme activity changes on intracellular metabolite dynamics in AGE1.HN.AAT cells

Abstract

Optimization of bioprocesses with mammalian cells mainly concentrates on cell engineering, cell screening and medium optimization to achieve enhanced cell growth and productivity. For improving cell lines by cell engineering techniques, in-depth understandings of the regulation of metabolism and product formation as well as the resulting demand for the different medium components are needed. In this work, the relationship of cell specific growth and uptake rates and of changes in maximum in vitro enzyme activities with intracellular metabolite pools of glycolysis, pentose phosphate pathway, citric acid cycle and energy metabolism were determined for batch cultivations with AGE1.HN.AAT cells. Results obtained by modeling cell growth and consumption of main substrates showed that the dynamics of intracellular metabolite pools is primarily linked to the dynamics of specific glucose and glutamine uptake rates. By analyzing maximum in vitro enzyme activities we found low activities of pyruvate dehydrogenase and pyruvate carboxylase which suggest a reduced metabolite transfer into the citric acid cycle resulting in lactate release (Warburg effect). Moreover, an increase in the volumetric lactate production rate during the transition from exponential to stationary growth together with a transient accumulation of fructose 1,6-bisphosphate, fructose 1-phosphate and ribose 5-phosphate point toward an upregulation of PK via FBP. Glutaminase activity was about 44-fold lower than activity of glutamine synthetase. This seemed to be sufficient for the supply of intermediates for biosynthesis but might lead to unnecessary dissipation of ATP. Taken together, our results elucidate regulation of metabolic networks of immortalized mammalian cells by changes of metabolite pools over the time course of batch cultivations. Eventually, it enables the use of cell engineering strategies to improve the availability of building blocks for biomass synthesis by increasing glucose as well as glutamine fluxes. An additional knockdown of the glutamine synthetase might help to prevent unnecessary dissipation of ATP, to yield a cell line with optimized growth characteristics and increased overall productivity.

Keywords: Central carbon metabolism; Enzyme activity; Intracellular metabolite; Mammalian cell; Mathematical model; Recombinant protein.